Nervous and hormonal mechanisms in the aging process

Abstract

An attempt is made to differentiate nervous and hormonal mechanisms in the development of atrophy and involution. Four different main types of muscle atrophy (due to: (a) loss or reduction of the trophic influence of the nerve cell, (b) disuse, (c) hormonal-especially gonadal hypofunction and (d) aging) are distinguished and it is shown, that senile muscle atrophy presents specific features with different mechanisms operating. Senile muscle atrophy is characterized by a decrease of transmitter (ACh) release, unchanged sensitivity to ACh, decrease of speed of contraction (with decrease of myosin ATPase activity) and maximal rate of tension development. Atrophy, due to gonadal hypofunction is essentially a disuse atrophy in the target organs showing e.g. decrease of speed of contraction but no change in transmitter release. Comparison of the changes in speed contraction time due to denervation, castration and old age demonstrate that it is possible to differentiate the different types of atrophy. Senile muscle atrophy combines features of both, disuse and decrease of the trophic function of the nerve cell. However, the decrease of protein synthesis in neurons of old animals, some analogy to denervation changes in the muscle and the reduction of axonal flow and of transmitter release suggest, that old age changes are primarily the result of a decline in the trophic function of the nerve cell, potentiated by the effect of disuse and gonadal hypofunction. The basic differences between senile involution and involution during development and the neural and hormonal components in the involution process are discussed.